When running a roll-to-roll coating line, you’ve probably asked one of two questions: “how fast can I run it?” or “why won’t my existing line go any faster?” We answer both questions several times a month, and in both cases the honest answer is the same, there’s no universal upper limit.
The ceiling on drying throughput depends on a number of variables, including the coating, the substrate, and the dryer. And it can change from one job to the next.
In this guide, rather than providing a figure that won’t actually apply to your process, we’ll run you through the framework for working out the ceiling for your line.
Here’s what we’ll cover:
- What actually limits drying speed
- Residence time – the trade between dryer length and line speed
- Drying technologies and what each is actually good for
- Why going faster usually means going longer, not hotter
- The constraints people forget
- Working out your own ceiling – a step-by-step framework
- When the honest answer is a trial rather than a calculation
What actually limits drying speed
With drying limits, it helps to think about the process rather than the machine. The rate is determined by three main factors: how much liquid has to leave, how fast it can evaporate, and how much heat the substrate can tolerate. We explore each and its impact below.
Coat weight and wet film thickness
Or, how much liquid has to leave. This is the first thing that sets the duty – the particular combination of coating, substrate and coat weight running in your line.
The determining factor here is how much solvent or water needs to be removed, and this amount scales with wet coat weight. The formula:
- Volatile load per square metre = wet coat weight x (1 – solids fraction)
Working through an example: for a 20 g/m² wet coat at 40% solids, a dryer has to remove 20 x (1 – 0.4), or 12 g/m² of solvent.
Thickness impacts drying speed because thicker wet films dry more slowly per unit mass. If not left to dry adequately, the surface can form a skin which traps solvent, leading to blistering, retained solvent and weak adhesion, as well as slower drying.
Solvent or water load and evaporation rate
Different volatiles – the solvent or water that has to evaporate – leave surfaces at different rates and at different energy costs. Two factors matter here:
- Vapour pressure, or how readily the volatile evaporates
- Latent heat of vaporisation, or how much each kilo needs
Water-based systems often need more drying energy and residence time because water has a higher latent heat than most organic solvents. Evaporation rate also depends on variables like how much solvent is in the air, the temperature of the air, and its velocity.
Substrate temperature limits
Each substrate has an innate limit on how hard you can push it during the drying process. Thin polymer films shrink or distort above a certain limit. Heat-sensitive coatings degrade. Knowing the properties and limits of each substrate is a crucial ingredient for configuring the drying process.
This is why just adding more heat to the process usually isn’t a viable option. Doing so without regard for the properties of the substrates involved can quickly lead to damage rather than quicker drying – despite this feeling like the intuitive outcome.
Residence time – the trade between dryer length and line speed
Residence time is the amount of time any point on the web spends inside the dryer. It is calculated by dividing dryer length by line speed.
Every duty needs a minimum amount of residence time to remove the volatile load within the substrate’s temperature limit. Fall below the time and the coating will still be wet when it leaves the dryer.
If you want to run faster, there are only two ways to do this:
- Extend the line: at a fixed line speed, residence time increases in proportion to length. Double the length and you double the time each point on the web spends in it.
- Raise the drying rate: the same volatile load clears in less time, meaning lower residence time required
The issue here is that the second option has a hard limit: you can only increase the drying rate so far before it bumps up against the substrate’s temperature limit, the coating’s tendency to skin, and for some systems, the LEL (more on this later). When you reach this point you can’t push harder without causing damage – meaning that more throughput usually requires a longer dryer rather than a hotter one.
Drying technologies and what each is actually good for
In this section we introduce the relative strengths and limitations of convection and infrared – the two main ways to heat a coating. Most real lines use some combination of both, meaning to tap into the strengths of each while offsetting each other’s limitations.
Convection and air flotation
This method uses hot-air impingement to blast surfaces with high-velocity jets of heated air, and to cut through the insulating boundary layer of cold air. Drying rates rise with air temperature and velocity.
Flotation dryers support the web on cushions of air rather than rollers, meaning no direct contact with wet coating, gentler handling, and the ability to dry both sides at once.
Strengths:
- Even, predictable drying across wide webs, over a broad range of coatings and solvents
- Air flotation dries without touching the wet face, good for delicate webs and double-sided coating
- Lowers defect risk
Limitations:
- High energy requirements
- Rate is bound by air temperature and velocity, both capped by the substrate limit and LEL in solvent systems
Infrared
Infrared applies radiant heat directly into the coating and substrate, heating it up very quickly. This is effective in raising the early constant-rate period, rather than as a standalone solution.
This is a precision tool: the infrared wavelength can overheat a substrate if it isn’t controlled properly, leading to distortion, shrinkage, or coating defects.
Strengths:
- Very fast: provides high heat-up and power in a short length
- No contact and easily zoned
Limitations:
- Effect dependent on the coating and substrate’s absorption spectrum, and easy to overheat
- Not often effective as a standalone solution
Zoned and combination approaches
By using zones with independently controlled air and temperature, you can follow the drying curve more closely. A gentle first zone avoids skinning, followed by a hotter middle where the majority of the solvent leaves, then a cooler stage for setting.
This combination lets you make the most of both drying methods without bumping up against their limitations, for instance: you don’t exceed the ceiling of the substrate when drying it with infrared.
In short: controlled staging is where drying speed can meaningfully increase.
Strengths:
- Lets you follow the drying curve and extract the most rate within the hard limits
- Ability to run different duties on the same line
Limitations:
- More complex setup and control
- Higher initial investment and a larger footprint
Why going faster usually means going longer, not hotter
When you bump up against the thermal ceiling of the substrate, or the temperature where skinning and defects or more likely, adding more heat becomes unviable – regardless of how tempting and intuitive it might seem.
Beyond this ceiling, the extra head causes damage rather than drying it faster. The focus shifts to more effective drying, not quicker. And ways to achieve this include:
- A longer dryer
- More and better-tuned zones
- Smarter airflow
- An IR boost
This is why asking “how can I run my existing line faster?” usually leads to a conversation about whether the dryer can be extended, whether zones and airflow can be reworked, and so on – rather than just a one-setting answer.
The constraints people forget
When designing lines and planning drying processes, some constraints are more obvious than others. In addition to those we’ve explored already, here are some constraints that can easily be forgotten, along with their implications:
- LEL headroom: exhaust air must stay safely below the Lower Explosive Limit – a ceiling which caps how much solvent the air can carry, itself a limiting factor on evaporation rate and recirculation. These place direct limits on throughput. If flammable solvent is used, we need to monitor the dryer in some way to ensure we do not exceed LEL limits. Dryer temperature and solvent throughput determine the type of monitoring required.
- Exhaust and solvent handling: your line needs enough exhaust volume and recovery capacity to match the duty. If this is too low, the dryer can’t run at optimum rate.
- Floor space and building layout: a longer dryer takes up more space, whether direct floor length or vertical space for vertical configurations. Floor space, floor load weight, ceiling height – all constrain what you can realistically and safely install.
Working out your own ceiling – a step-by-step framework
By using four numbers you already have or can readily acquire, it’s possible to put a realistic ceiling on your current line. Here are the steps to do it.
- Work out how much has to leave: multiply your wet coat weight by (1 – solids fraction) to get the volatile load per square metre. This is the mass the dryer needs to remove.
- Find the substrate’s temperature ceiling: either from the supplier’s datasheet or from trials. This caps how hard you can drive drying.
- Establish the drying rate you can achieve within the ceiling (and within the LEL for solvent systems). This figure can be hard to arrive at by calculation, and is where trial data comes in useful.
- Divide the load by the rate: this is the residence time the duty needs
- Divide your dryer length by the residence time: this gives the line speed it will support
- Sanity check and iterate
When the honest answer is a trial rather than a calculation
There are limits to what calculations and frameworks can achieve. With novel coatings or substrates, drying curves can be hard to predict. Factors like rheology, multi-solvent blends, skinning behaviour and adhesion interact in complex ways that calculations can’t accurately capture.
This is why pilot lines are often the right move: a short trial lets you measure the real drying curve instead of inferring it. You can find the rate ceiling, the skinning point and other variables on small amounts of material, rather than committing to a full production without knowing them.
This is what our Genesis Air Lab Series platform is built for: a lab-scale roll-to-roll coating and drying line where you can characterise a coating’s drying behaviour and carry what you learn through to pilot and production scale.
In conclusion
The framework above gives you the ceiling for how fast you can dry a coating within the limitations of your line, along with ways to adjust things for faster drying. If you’ve got questions, drop us a message and we’ll be happy to help. Or if you’d prefer to discuss accessing time on our equipment for your product development or prototyping, take a look at our Technology Centre.
FAQs
How fast can you dry? This depends on the duty and the following factors: how much volatile has to leave, how fast it can leave within the substrate’s temperature limit, and how much dryer length you have.
Can I run faster without making the dryer longer? Yes, but only until you hit the substrate’s thermal ceiling or the coating starts to skin. Beyond that more heat will cause damage.
Will more heat damage my substrate? Past a point, yes. Thin films can soften or distort.
How long does a dryer need to be? There is no one-size-fits-all answer – the length needs to be enough to deliver the required residence time at your target speed.
